1. Field of the Invention
This invention relates to compounds useful as stabilizers in photothermo-graphic elements.
2. Background of the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) which are developed with heat and without liquid development have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively or completely non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example, for the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
In photothermographic elements, the photosensitive compound is generally a photographic type photosensitive silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photosensitive silver halide, those nuclei are able to catalyze the reduction of the reducible silver source within a catalytic sphere of influence around the silver specks. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions (see, for example, Research Disclosure, June 1978, Item No. 17029).
The silver halide may be made "in situ," for example by adding a halogen-containing source to a reducible silver source to achieve partial methasis and thus causing the in-situ formation of silver halide (AgX) grains throughout the silver soap (see, for example, U.S. Pat. No. 3,457,075).
The silver halide may also be pre-formed and prepared by an ex situ process whereby the silver halide (AgX) grains are prepared and grown in an aqueous or an organic solvent. It is reported in the art that when silver halide is made ex situ, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The silver halide grains prepared ex-situ may then be added to and physically mixed with the reducible silver salt.
A more preferable method is to prepare the reducible silver salt in the presence of the ex-situ prepared grains. In this process, the pre-formed grains are introduced prior to and are present during the formation of the silver soap. Co-precipitation of the silver halide and reducible silver source provides a more intimate mixture of the two materials (see, for example, M. J. Simons U.S. Pat. No. 3,839,049).
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic compounds, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photo-sensitive emulsion must be further developed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). This produces a black-and-white image. In photographic elements, the silver halide is reduced to form the black-and-white negative image in a conventional black-and-white negative imaging process. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white negative image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic elements, the reducing agent for the silver ion of the light-insensitive silver salt, often referred to as a "developer," may be any compound, preferably any organic compound, that can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature above 100.degree. C. At elevated temperatures, in the presence of the latent image, the silver ion of the non-photosensitive reducible silver source (e.g., silver carboxylate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photo-thermographic formulations and fog during preparation and coating of photo-thermographic elements. As a result, hindered phenol developers (i.e., reducing agents) have traditionally been preferred.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.
In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development. Temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths). Development is usually performed at a more moderate temperature (e.g., about 30.degree. C. to about 50.degree. C.).
In photothermographic elements, only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver carboxylate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the physical development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed, black-and-white photographic elements use only one form of silver (e.g., silver halide); which, upon chemical development, is itself converted to the silver image; or which upon physical development requires addition of an external silver source. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as a silver carboxylate, which participates with the developer in developing the latent image. In contrast, chemically developed photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (e.g., silver carboxylate) while the image in photo-graphic black-and-white elements is produced primarily by the silver halide.
In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic elements can lead to increased formation of various types of "fog." Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon preparation of the photothermographic emulsion as well as during coating, storage, and post-processing handling of the photothermographic element.
In photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic elements, the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.), which are intended to have a direct effect upon the imaging process, can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.
Because of these and other differences, additives which have one effect in conventional silver halide photography may behave quite differently in photo-thermographic elements where the underlying chemistry is so much more complex. For example, it is not uncommon for an antifoggant for a silver halide system to produce various types of fog when incorporated into photothermographic elements.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition), J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989, Chapter 9; in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978, pp. 74-75; and in C-f Zou, M. R. V. Shayun, B. Levy, and N Serpone J. Imaging Sci. Technol. 1996, 40, 94-103.
Fog in Photothermographic Elements
Various techniques are typically employed to try and gain higher sensitivity in a photothermographic element. In efforts to make more sensitive photothermo-graphic elements, one of the most difficult parameters to maintain at a very low level is the various types of fog or Dmin. Fog is spurious image density which appears in non-imaged areas of the element after development and is often reported in sensitometric results as Dmin. Photothermographic emulsions, in a manner similar to photographic emulsions and other light-sensitive systems, tend to suffer from fog.
Photothermographic elements can suffer from fog during preparation and storage of the photothermographic emulsion. This is referred to as "pot-life" fog. In addition photothermographic elements can suffer an increase in fog upon coating and drying of the of the photothermographic element. This is referred to as "coating" fog. The fog level of freshly prepared photothermographic elements caused by "pot-life" fog and coating fog will be referred to herein as initial fog or initial Dmin.
In addition, the fog level of photothermographic elements often rises as the element is stored, or "ages." This type of fog will be referred to herein as shelfaging fog. Adding to the difficulty of fog control on shelf-aging is the fact that the developer is incorporated in the photothermographic element. A great amount of work has been done to improve the shelf-life characteristics of photothermographic elements.
A third type of fog in photothermographic systems results from instability of the image and/or background after processing. The density of the image or the Dmin of non-imaged areas continues to increase with time. This type of fog is known variously as "print instability," "post-processing fog," or "silver print-out." One cause of post-processing fog is from the photosensitive silver halide still present in the developed image continuing to catalyze formation of metallic silver.
Another cause is from the decomposition of other materials in the photothermo-graphic element such as sensitizers, antihalation materials, stabilizers, etc. Post-processing fog often occurs from prolonged room light handling. It can be particularly severe if imaged and developed photothermographic elements are left on a light box; are stored for a prolonged period of time as, for example, during transport in a hot vehicle by a courier service or a patient; or are used as photomasks and require post-processing exposure such as in graphic arts contact frames.
In color photothermographic elements, often unreacted dye forming or dye releasing compounds may slowly oxidize and form areas of color in the unexposed areas. In these elements, stabilizers are often added to reduce "leuco dye back-grounding."
U.S. Pat. No. 5,686,228 describes the use of propenenitrile compounds as antifoggants for black-and-white photothermographic and thermographic elements. U.S. Pat. No. 5,460,938 describes the use of 2-(tribromomethylsulfonyl)quinoline as an antifoggant in photothermographic elements. 2-(4-Chlorobenzoyl)benzoic acid, benzotriazole, and tetrachlorophthalic acid, have also been used as antifoggants in photothermographic elements.
There is a continued need for improved stabilizer compounds that inhibit all types of fog and do not have any detrimental effects on the photothermographic element.